Ferroelectric ceramics are currently the material of choice for ultrasonic transducer applications. These materials offer electromechanical coupling factors (kij) in a range between about 60% and about 70% and a wide range of dielectric constants (K). These characteristics translate into transducer performance in the form of relatively high sensitivity and broad bandwidth.
One example of a ferroelectric ceramic is lead zirconate titanate (Pb(Zr1-x,Tix)O3, also called PZT. PZT is a ceramic perovskite material that shows a marked piezoelectric effect. Currently, PZT ceramics are the mainstay for high performance transducer applications. PZT ceramic compositions lie near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases. These PZT ceramics have anomalously high dielectric and piezoelectric properties as a result of enhanced polarizability arising from coupling between two equivalent energy states, i.e. the tetragonal and rhombohedral phases, allowing optimum domain reorientation during the poling process. PZT ceramics may be modified using acceptor dopants to form acceptor modified PZT ceramics having a high mechanical quality factor Q.
Acceptor modified PZT ceramics, such as DOD Type I & III piezoelectric ceramics, are often used in high power applications. These materials exhibit hard ferroelectric characteristics, or in other words, have a mechanical quality factor Q (inverse of mechanical loss) in the range of about 500 to about 1500 and a dielectric loss of about 0.4%. However, these materials also have a low dielectric permittivity in the range of about 1000 to about 1500 and low electromechanical coupling factors, k33, of less than 70%.
Alternative MPB systems are relaxor-based ferroelectrics include their solid solutions with PbTiO3 (PT). In these relaxor-based ferroelectrics, the transition between piezoelectric behavior and loss of piezoelectric capability does not occur below a specific temperature (for example, the Curie point for PZT ceramics), but instead occurs over a temperature range. These lead based relaxor materials exhibit a complex perovskite-type crystal structure with the general formula Pb(B1B2)O3, where B1 may be selected from the group including Mg2+, Zn2+, Ni2+, Sc3+, In3+, Yb3+, and B2 may be selected from the group including Nb5+, Ta5+, and W6+, and have compositions that lie near an MPB. These relaxor-PT ceramics can be formulated to have superior dielectric and piezoelectric properties compared to most PZT ceramics it is usually done at the expense of temperature stability. If analyzed with respect to the ferroelectric transition temperature (the temperature at which the material transforms from the prototypical non-ferroelectric to ferroelectric phase being associated with a spontaneous polarization and large dielectric anomaly), no one type of ceramic, relaxor-PT or PZT, offers significant advantages in overall performance.
Though relaxor-PT ceramics do not offer enhanced dielectric and piezoelectric properties comparable to PZT ceramics of similar transition temperatures, it is the single crystal form of relaxor-PT ceramics that exhibit ultrahigh piezoelectric properties not currently available with piezoelectric MPB ceramics. Relaxor-PT single crystals, such as Pb(Zn1/3Nb2/3)O3—PbTiO3 (PZNT) and Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMNT), have excellent properties in the <001> poled orientation. These relaxor-PT single crystals have high piezoelectric coefficients d33 in the range of greater than 1500 pC/N and large dielectric permittivity ∈r in the range of about 5,000 to about 7,000, due to a ferroelectric domain engineered configuration in the material. However, these materials also have a relatively low mechanical quality factor (Q) of less than about 100. The combination of high piezoelectric properties and low Q makes relaxor-PT single crystals attractive for non-resonant actuators and high frequency medical ultrasound transducer applications. However, these characteristics are not desirable for high power applications, such as medical ultrasonic and high duty cycle sonar transducers, which require a transducer having a high mechanical quality factor (Q) in order to reduce heat generation. Medical ultrasonic applications may include high intensity focused ultrasound (HIFU) and ultra sound-guided HIFU therapy, which require transducers capable of both high quality diagnostic imaging and radiating high acoustic power into tissue.
Therefore, there is a need for a ferroelectric single crystal having a high dielectric permittivity and high electromechanical coupling factor (kij) to improve sensitivity and bandwidth, and having a high mechanical quality factor (Q) to meet the requirement of delivering higher power.